Description (Adapted from Application): Members of one family of larger foraminifera, the Soritidae, are the hosts for a wide variety of unicellular endosymbiotic algae. It is reasonable to speculate that they have some symbiotic recognition and maintenance systems in common, and some of which are more specific. One interpretation is that, over time, the hosts an symbionts have maintained wide flexible relationships. This idea is contrary to a paradigm derived from studies of terminates and their parabasalian endosymbionts, which suggest that hosts and symbionts co-evolved. The investigator has identified an interesting evolutionary and systemic problem that suggests an alternative symbiotic evolutionary pathway that has potential for probing in the laboratory for probing in the laboratory. The study has the potential to show that a system of host/symbiont relationships that are not finical could have advantages for survival and adaptation in changing habitats. Tests of the speculation that soritid foraminifera acquire(d) dinoflagellate endosymbionts from environmental pools contributed to by coelenterates in their habitat are proposed. The aims of this proposal are: (1) to analyze the dinoflagellate species structure in populations of soritid larger foraminifera; (2) to compare the foraminiferal dinoflagellate endosymbiont species at collection sites with those of nearby corals and other invertebrates; and (3) to search for common surface antigens which may be involved in the initial signaling process and maintenance. Since this aspect of cellular evolution is highly developed in foraminifera, it is hoped that they will provide rich new models for the study of molecular interactions between phagotrophic unicellular eukaryotes and prey which has developed the ability to escape digestion.